By landing on the last casing hanger and locking into the subsea wellhead running profile, or into the horizontal tree grooves, the lockdown sleeve (LDS) preloads all of the wellhead components into the bottom of the wellhead. This minimizes the stress induced from thrust loads and thermal loads on the wellhead system throughout the life of the system, and increases the useful service life. An LDS with a seal has been set using a drilling riser with extreme weight or hydraulic pressure from the BOP stack, as discussed below.
The LDS is typically run through the riser and landed on the top of the upper casing hanger. The rams on the BOP are closed to apply pressure to the LDS seal. After the LDS is landed, pressure is applied and the seal is tested from above. Then hydraulic pressure is applied to the drill pipe to actuate a sleeve on the running tool that locks the LDS to the wellhead. The rams are opened and the running tool is removed by a straight pull which shears spring loaded shear pins.
The lockdown sleeve or LDS is used to lock down the casing hanger from thermal growth and protect the bore of the casing head in the event of drilling through it. The preferred LDS has a seal on the lower end and a seal pocket in the upper end along with a shear pin groove for attachment of the running tool. The LDS has typically been run inside the BOP via the running tool.
Another LDS tool is run in open water by drill pipe without a BOP stack. By running on drill pipe, the weight to set the seal is significant. To achieve this weight without hydraulic pressure, drill collars may be added to the drill pipe string above and below the running tool to achieve the weight needed to set the seal. After the seal is set, the LDS may be locked by hydraulic pressure applied via the drill pipe to the running tool to lock the LDS to the wellhead housing. After the LDS is locked in place, pressure may be applied to test the LDS seal from below. The seals in the running tool and the cup tester have the same diameter and are pressure balanced. Once the seal has been tested, the running tool may be removed by shearing the shear pins for retrieval with straight pull.
The LDS running tool may be attached by shear pins in a groove above the seal pocket in the upper end of the LDS. The running tool may have lock, unlock and test functions. A test sub may be attached at the bottom of the tool for testing the seal along with the test cup for sealing in the upper casing hanger and casing.
The cost of running the LDS inside the BOP is very high and takes a dedicated riser and drilling vessel. Although the LDS may be run on drill pipe, this would also take a dedicated vessel and drill pipe, and this technique takes extreme weight to set the seal. To achieve this weight, heavy and expensive drill collars are added to the string.
There is thus a need for an improved running tool for use with a lockdown sleeve to preload wellhead components into the bottom of a subsea wellhead.